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Re: Attack on badly configured Netfilter-based firewalls
From: Solar Designer <solar () openwall com>
Date: Mon, 27 Feb 2012 01:53:29 +0400

On Sun, Feb 26, 2012 at 10:05:55PM +0100, Eric Leblond wrote:
On Sun, 2012-02-26 at 12:17 -0700, Kurt Seifried wrote:
Are there any helpers that can be abused to open holes in the firewall
externally, or is it only internal clients that can cause problems and
trigger the firewall to improperly allow network traffic in/out.

No, attacker has to be on a network directly connected to the firewall.

I guess by "internal clients" Kurt was referring to machines behind the
firewall (e.g., someone clicking an URL that has a string looking like
an FTP command embedded in it, thereby triggering the FTP helper to open
a hole - stuff that was discussed in late 1990s and partially mitigated
by hardening the helpers at the time), whereas by "attacker on a network
directly connected to the firewall" Eric means that the attacker may be
_outside_ the firewall (behind its WAN interface), but on the same
network segment (e.g., the attacker might have compromised a nearby
server, such as of another customer at a colocation facility).

It is known that a machine will generally receive and process a packet
routed to one of its NICs by MAC address even if the destination IP
address is that of another NIC or even loopback (e.g., it is possible to
access services bound to 127.0.0.1 in this way - but only from directly
connected machines).  Without rp_filter or equivalent, it is possible to
have these packets' source addresses match the other NIC's network
segment.  My _guess_ (based solely on the info posted in here so far) is
that the gist of Eric et al.'s new attack is to apply this approach
against a protocol helper.  The novelty is thus in combining these known
things together to arrive at something that to the best of my knowledge
has not yet been discussed.

I suppose Eric will tell us if this is the correct guess or not. ;-)

Alexander


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